Can my ADU's electrical line share a trench with water or gas lines?

Often, yes. For private lines on your own property, some California building departments allow electrical conduit, gas piping and water piping in a common trench without horizontal separation, as long as each line meets its own minimum depth, while water and sewer lines sharing a trench have their own separation rules. Your local building department, and your utility for any utility-owned line, has the final say.

What a shared trench typically looks like

The City of Ojai's homeowner handout on burial depths is a good example of how this works. It says yard piping and electrical lines "may be laid in the same trench without horizontal separation except as noted for sewer and water lines," and it lists each line's own requirements:

• Electrical: depth per the electrical code's cover table, such as 18 inches over PVC conduit and 24 inches over direct-burial cable.
• Plastic gas piping: needs a copper tracer wire so it can be located later.
• Water and sewer together: the water pipe sits on a solid shelf at least 12 inches above the sewer and at least 12 inches to the side.

When the rules get stricter

• Utility-owned lines. If your ADU gets its own electric or gas service, the utility's standards govern its service line, and Ojai's handout notes that its yard depths do not apply to utility company gas and electric lines.
• Local amendments. Some jurisdictions set their own spacing between electrical and gas lines or between electrical and plumbing. Check before you design the trench, not after it is dug.
• Different depths. Each line must still meet its own cover, so the line needing the most cover sets how deep that part of the trench goes.

Practical tips

• Lay out the sewer first. A gravity sewer has to fall continuously, so it often sets the trench profile, and the other lines can follow it.
• Keep the electrical route straight and documented so anyone digging later knows where it is.
• Use warning tape above electrical lines, even where the code does not require it.
• Photograph the open trench with a tape measure in view before backfill, for your records.

What this means for your electrical plan

Your site plan should show each trench route, and a trench section detail should show every line in it with its depth and spacing, so the electrical, plumbing and gas drawings agree with each other. See sharing plumbing with the main house, gas line requirements for ADUs and ADU sewer connections for the other lines that usually share the yard.

Sources: ojai.ca.gov, pge.com, webprod.ladwp.com.